Rust. I live in the north half of Ohio, so my stuff gets bathed in salty brine for several months out of the year and rust is a real problem for me.
What I've found that works (for me):
For stuff that isn't yet rusted, Fluid Film. It's easy to buy (it's on the shelf even at Wal-Mart). It's made primarily from lanolin, which is a product of the wool industry and is how sheep stay dry. If I were Very Serious about it, I'd find a shop that would cover the whole bottom of the vehicle (and anything that can be reached through holes) in the stuff and pay them to get that done. (I buy it in spray cans; some shops buy it in 55 gallon drums.)
For stuff that is definitely already rusting, Corrosion-X. It's some kind of oily chemical soup that is supposed to prevent existing rust from getting worse, and also prevent new rust. One interesting feature is that it's available in 3 different viscosities; vaguely speaking, those viscosities are thin, medium, and elephant snot.
The thin one does a fantastic job of creeping around to cover even unseen surfaces, but it washes off the fastest. The thicker ones hang around longer and creep less. (Tradeoffs, I guess.)
I prefer Fluid Film just because it's more natural than some other things are and that makes me feel good in some way that I don't care to rationalize, but Fluid Film is not very good at recovering from existing rust.
Corrosion-X, though? I can get the thin version of that worked into the joint of a completely rusted-stuck pair of box-jointed pliers and have them working very well (and looking fairly decent, though not "new") in a few minutes with a shop rag. I've heard stories of it being used to hose down whole electrical rooms in ocean-going boats. It's amazing stuff. (And it's expensive.)
The practical downside is that these products all feel greasy, and they all turn black with enough time and enough miles. They're all ugly.
For visible painted body panels, the best way I know to deal with small spots of rust from rock chips and stuff is to go full-ass on it. Get the Dremel out, pick an appropriate abrasive stone, and start grinding those little pinholes out until there's nothing but clean, shiny metal surrounded by paint. And then: Fill in with touchup paint that matches the factory paint code. (It's never perfect, but it does get easier to do a job that looks better than little rust spots do with some practice...and the little spots then don't turn into big spots.)
There are many shops in the US which will apply Noxudol both underneath and inside body panels and frame rails with special 360 degree applicators. I believe it is a formula developed in Scandinavia.
All of my cars are sprayed with the stuff for over a decade with no other maintenance.
Perhaps, but: I don't want to deal with avoiding permanently-affixed overspray on parts that don't want to be coated.
I also don't want to work with fasteners that are coated in bedliner: I'm already not having a fun time of things when I'm crawling under an old car doing some manner of repair. I want every possible advantage while I'm down there, and a well-stuck layer of bedliner seems like a big disadvantage.
As a point of comparison, stuff like Fluid Film [and the others that have been mentioned] can be applied to just about anything under the car that's metal (including bendy things like springs), and can be scrubbed off sometime later if it accidentally gets on body-colored parts using just soap, water, and some elbow grease.
Fasteners that are both rust-free and oily usually come apart like a dream when the time comes, and oily coatings that stay goopy tend to self-heal after being abraded by whatever the tires might kick up from the road.
Fluid coatings seem like the right set of tradeoffs in this non-ideal world compared to something like bedliner.
They're not perfect, but nothing is.
(Ideally, I'd live in a place that doesn't require driving through brine... but my world isn't ideal.)
I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["elephant snot"], and perhaps I could never succeed in intelligibly doing so. But I know it when I see it.
What I've found that works (for me):
For stuff that isn't yet rusted, Fluid Film. It's easy to buy (it's on the shelf even at Wal-Mart). It's made primarily from lanolin, which is a product of the wool industry and is how sheep stay dry. If I were Very Serious about it, I'd find a shop that would cover the whole bottom of the vehicle (and anything that can be reached through holes) in the stuff and pay them to get that done. (I buy it in spray cans; some shops buy it in 55 gallon drums.)
For stuff that is definitely already rusting, Corrosion-X. It's some kind of oily chemical soup that is supposed to prevent existing rust from getting worse, and also prevent new rust. One interesting feature is that it's available in 3 different viscosities; vaguely speaking, those viscosities are thin, medium, and elephant snot.
The thin one does a fantastic job of creeping around to cover even unseen surfaces, but it washes off the fastest. The thicker ones hang around longer and creep less. (Tradeoffs, I guess.)
I prefer Fluid Film just because it's more natural than some other things are and that makes me feel good in some way that I don't care to rationalize, but Fluid Film is not very good at recovering from existing rust.
Corrosion-X, though? I can get the thin version of that worked into the joint of a completely rusted-stuck pair of box-jointed pliers and have them working very well (and looking fairly decent, though not "new") in a few minutes with a shop rag. I've heard stories of it being used to hose down whole electrical rooms in ocean-going boats. It's amazing stuff. (And it's expensive.)
The practical downside is that these products all feel greasy, and they all turn black with enough time and enough miles. They're all ugly.
For visible painted body panels, the best way I know to deal with small spots of rust from rock chips and stuff is to go full-ass on it. Get the Dremel out, pick an appropriate abrasive stone, and start grinding those little pinholes out until there's nothing but clean, shiny metal surrounded by paint. And then: Fill in with touchup paint that matches the factory paint code. (It's never perfect, but it does get easier to do a job that looks better than little rust spots do with some practice...and the little spots then don't turn into big spots.)
Rust never sleeps. Good luck.